March 7, 2011 -- Toenail tests tell whether you're at high risk of getting lung cancer from cigarette smoke, even if you're not a smoker.

The finding comes from the toenails of 210 men with lung cancer and a comparison group of 630 men without lung cancer enrolled in the Health Professionals Follow-Up Study. Most of the 33,737 medical professionals in this long-term study donated toenail clippings in 1987.

The 20% of toenails containing the highest amounts of nicotine identified men at the highest risk of lung cancer. These men were 10.5 times more likely to have lung cancer than the 20% of men with the least nicotine in their toenails.

Even when taking into account reported smoking -- that is, when comparing men at similar levels of cigarette use -- men with the most nicotine in their toenails were over 3.5 times more likely to get lung cancer than those with the least toenail nicotine.

"Regardless of whether you are a smoker or a nonsmoker exposed to secondhand tobacco smoke, we can now better measure your exposure and predict your risk," study researcher Wael K. Al-Delaimy, MD, PhD, tells WebMD. Al-Delaimy is chief of the division of global health at the University of California, San Diego School of Medicine.

Why toenails? Al-Delaimy was looking for a way to evaluate secondhand smoke exposure. He first measured nicotine in hair. But while working at Harvard with study co-author Walter Willett, MD, DrPH, he learned that Willett and colleagues had collected a large number of toenail samples from medical professionals.

"Given that hair and toenails are formed of the same kind of tissue, I thought maybe we could analyze toenails for nicotine," he says. "Toenails can be stored at room temperature for years. And they represent nicotine exposure during the past year."

In fact, Al-Delaimy and colleagues have shown that toenail nicotine levels are closely linked to smoking status six years prior to collection of clipping samples. And they have also shown that toenail nicotine levels predict the risk of heart disease in women. Women with the highest toenail nicotine levels have a 42% higher risk of heart disease than do those with the lowest levels.

"We knew tobacco was harmful, but we are now learning it is even more harmful than we had previously measured," Al-Delaimy says. "We are getting a better estimate of the true risk of tobacco's lung effects. And this could be applied to other disease outcomes such as coronary heart disease, too."

The findings don't surprise tobacco and health expert Michael Eriksen, ScD, director of the institute of public health at Georgia State University. Eriksen was not involved in the Al-Delaimy/Willett study.

"As this study shows, evidence of exposure to smoke can be found throughout the entire body -- even the tips of your toes -- and that this exposure to smoke increases the risk of lung cancer," Eriksen tells WebMD via email.

Al-Delaimy and Willett report their findings in the American Journal of Epidemiology, published online ahead of print on March 2.